Brendan Eich was making a point about the tab key also being used to switch to different text fields and buttons. This makes it difficult to type in certain applications. A space doesn't have that issue.
About your point of tab being represented different on different systems: It will always be ascii 9, how it's draw does differ between text editors but I consider that one of it's strengths for programming. Everyone can configure what an indent looks like for them, it makes reading code easier.